1. Field of the Invention
The present invention relates to a system for estimating the location of a wireless terminal on the basis of received wireless signals. More particularly, the present invention relates to such a system for high accuracy location estimation.
2. Description of the Background Art
A conventional system for location estimation includes a wireless terminal, the location of which is to be estimated, i.e. a target terminal, and three or more wireless terminals, the locations of which are known from the outset, i.e. reference terminals, which are adapted to transmit packets to respective neighboring terminals. The packets include identification codes proper to the respective terminals. On receipt of the packets, the respective terminals determine that the transmitting terminals stay within the range of possible signal delivery, and accordingly add the transmitting terminals to a list of neighboring terminals they have. When the transmissions between the totality of the terminals have come to an end, the lists of neighboring terminals are collected from those terminals to a location estimation device (sink terminal) and stored in the latter. The location estimation device progressively uses the obtained relationship of interconnection of the terminals and the locations of the reference terminals to narrow down the sphere of possible presence of the target terminals. More specifically, the system for location estimation deems that the distance between terminals, between which a packet has been transmitted with success, is less than the distance which allows for signal delivery, while deeming that the distance between terminals, between which a packet has not been transmitted with success, is not less than the distance which allows for signal delivery. The system for location estimation accordingly determines the distance between terminals in two stages, that is, a distance longer than the range of possible signal transmission and a distance not longer than the range of possible signal transmission, in order to effect location estimation. This is disclosed, for example, in N. Sundaram and P. Ramanathan, “Connectivity Based Location Estimation Scheme for Wireless Ad Hoc Networks”, IEEE Globecom 2002, pp. 143-147.
It is also known to model the relationship between the distance between the terminals with the power of received signals with the fading occurring on a wireless transmission path taken into account, and to estimate the location of a target terminal from the location of a plural number of reference terminals and an estimated distance between the target terminal and the reference terminals. It is likewise known to effect signal reception a number of times to diminish the effect of fading to improve the accuracy in location estimation. Those are disclosed, for example, in U.S. Pat. No. 6,473,038 to Patwari, et al., and Kentaro Yanagihara, et al., “A Maximum Likelihood Estimation Method of Localization using RSSI in Wireless Sensor Networks”, Technical report of IEICE, the Institute of Electronics, Information and Communication Engineers, IN2004-327. pp. 409-414.
However, with the conventional system for location estimation, if an area as the subject of location estimation becomes larger, then the more reference terminals have to be disposed. Moreover, if location estimation is to be improved in accuracy, it is necessary to install the reference terminals at a higher density to gather as much information as possible at the location estimation device. If the number of the target terminals is increased, the traffic of communication for location estimation is also increased. The amount of the information to be gathered is proportional to the product of the density of the reference terminals and the number of the target terminals. If the traffic of the communication for location estimation is increased, there is a possibility that the communication is deteriorated in quality. Hence, there is raised a demand for a system for location estimation may be moderated in which the problem associated with an increased communication traffic accompanying an increased sphere for location estimation and an increased number of target terminals, and at the same time high precision location estimation may be assured.
In addition, with the conventional system for location estimation, it is presupposed that the target terminals are immovable, so that, if target terminals are moving, location estimation accuracy may be lowered. Hence, there is raised a demand for a high precision system for location estimation capable of coping with a movable target terminal as a subject of location estimation.